Re: [PATCH v4 10/19] qapi/expr.py: Add casts in a few select cases

[John Snow <jsnow@redhat.com>]

On 3/25/21 10:33 AM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
> 
>> Casts are instructions to the type checker only, they aren't "safe" and
>> should probably be avoided in general. In this case, when we perform
>> type checking on a nested structure, the type of each field does not
>> "stick".
>>
>> (See PEP 647 for an example of "type narrowing" that does "stick".
>>   It is available in Python 3.10, so we can't use it yet.)
>>
>> We don't need to assert that something is a str if we've already checked
>> or asserted that it is -- use a cast instead for these cases.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>   scripts/qapi/expr.py | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/scripts/qapi/expr.py b/scripts/qapi/expr.py
>> index ca5ab7bfda..505e67bd21 100644
>> --- a/scripts/qapi/expr.py
>> +++ b/scripts/qapi/expr.py
>> @@ -15,7 +15,7 @@
>>   # See the COPYING file in the top-level directory.
>>   
>>   import re
>> -from typing import Dict, Optional
>> +from typing import Dict, Optional, cast
>>   
>>   from .common import c_name
>>   from .error import QAPISemError
>> @@ -259,7 +259,7 @@ def check_enum(expr, info):
>>   
>>   
>>   def check_struct(expr, info):
>> -    name = expr['struct']
>> +    name = cast(str, expr['struct'])  # Asserted in check_exprs
> 
> "Asserted" suggests an an assert statement.  It's actually the
> check_name_is_str() visible in the last hunk.  What about "Checked in
> check_exprs()" or "Ensured by check_exprs()"?
> 

I missed these. "Checked" is fine.